The present invention relates to a seat belt retractor, and more particularly, to a seat belt retractor wherein a force for withdrawing a seat belt is substantially uniform even when a withdrawing amount of the seat belt is increased.
Conventionally, in a seat belt retractor equipped in a motor vehicle, a seat belt (hereinafter referred to as a webbing in case of indicating a belt itself to be wound) is wound around a reel by spring driving force of a spiral spring or the like built in the seat belt retractor. In this type of the seat belt retractor, as a withdrawing amount of the webbing is increased, the spiral spring is wound so as to increase an urging-force thereof. Thus, there has been a situation that a force of withdrawing the webbing is increased as the amount of withdrawal of the webbing is increased.
To solve the aforementioned situation, a seat belt retractor has already been developed, which includes a tension reducing mechanism for reducing the force of withdrawing the webbing as disclosed in Japanese Patent No. 2,711,428. The tension reducing mechanism of the seat belt retractor is explained hereunder by referring to FIG. 7. As shown in FIG. 7, a belt reel 51 of a seat belt retractor 50 is axially supported by a base frame (not shown) through a rotational shaft 59. A webbing 57 is arranged to be wound on an outer peripheral surface of the belt reel 51. Further, a housing 61 for the tension reducing mechanism is fixed on an outer side wall of the base plate (not shown). A shaft end 59a of the rotational shaft 59 extends to an inside of the housing 61 attached adjacent to the base frame. A first rope pulley 52 in a shape of a truncated cone is fixed to the shaft end 59a extending inside the housing 61. A spiral guide groove 55 is incised on an outer peripheral surface of the first rope pulley 52.
Furthermore, a second rope pulley 54 is accommodated in the housing 61 in a state that the second rope pulley 54 is axially supported through a rotational shaft 62. As in the first rope pulley 52, the second rope pulley 54 is formed in a shape of a truncated cone and provided with a spiral guide groove 58 on an outer peripheral surface thereof. The rotational shaft 62 is disposed parallel to the rotational shaft 59 at a position laterally spaced away therefrom with a predetermined distance. As shown in FIG. 7, a hollow chamber 63, which has a cylindrical shape and is coaxial with the rotational shaft 62, is formed in the second rope pulley 54. The spiral spring 56 is situated inside the hollow chamber 63. One end of the spiral spring 56 is fixed to the second rope pulley 54 supported by the rotational shaft 62, and the other end of the spiral spring 56 is fixed to a bearing boss 65 formed in the hollow chamber 63 of the second rope pulley 54.
Further, a rope 53 extends between the first rope pulley 52 and the second rope pulley 54. In a condition that the webbing 57 is completely wound, one end of the rope 53 is fixed to a fixing end (not shown) at a largest diameter position of the guide groove 58 of the second rope pulley 54 such that the rope 53 is spirally wound in the entire periphery or length of the guide groove 58, and the other end of the rope 53 is fixed to a fixing end 67 of a largest diameter portion of the guide groove 55 of the first rope pulley 52.
In the tension reducing mechanism thus structured, an urging force for winding generated in case of winding up the spiral spring 56 is transmitted to the second rope pulley 54 as a rotational force or torque for winding the end portion of the rope 53 wound in the guide groove 58 of the second rope pulley 54. At this time, as shown in FIG. 7, the rope 53 extends from the smallest diameter portion of the second rope pulley 54 to the largest diameter portion of the guide groove 55 of the first rope pulley 52. When the webbing 57 in the wound state is withdrawn against a winding force shown by an arrow K in the figure, the first rope pulley 52 of the tension reducing mechanism rotates in a direction of an arrow A in the figure, so that the rope 53 is gradually wound on the first rope pulley 52. At this time, the rope 53 is wound on the first rope pulley 52 while being held in the guide groove 55. In accordance therewith, the rope 53 wound in the guide groove 58 is gradually unwound from the second rope pulley 54.
As described above, the rope 53 held in the guide grooves 55 and 58 respectively formed on the outer peripheral surfaces of the first rope pulley 52 and the second rope pulley 54 transmits the rotations of the rope pulleys 52 and 54 therebetween. In this case, by adequately setting the cone shapes of the first rope pulley 52 and the second rope pulley 54, a rotational ratio of the first rope pulley 52 to the second rope pulley 54 can be controlled, so that the force for withdrawing the webbing can be made constant or reduced regardless of a withdrawal amount of the webbing.
In the seat belt retractor shown in FIG. 7, however, since the tension reducing mechanism is formed of two rope pulleys in the shape of the truncated cone having the spiral guide groove, there have been a problem that assembling the rope pulleys with the housing is poor, and a problem that the miniaturizing the entire apparatus is difficult because the roller pulley itself can not be miniaturized.
Also, in the seat belt retractor, by winding the rope on the outer peripheral surface of the rope pulley in the shape of the truncated cone, the force for withdrawing the webbing is reduced with a long unwinding stroke. However, since the rope is wound several times on the outer peripheral surface of the truncated cone with a gentle lead angle, a ratio of decreasing a radius of the wound rope is small at the time of unwinding the rope, so that a change in the force of withdrawing the webbing can not be taken greatly.
Further, since the rope winding surface is inclined as a result of the shape of the truncated cone, it is necessary to form the deep guide groove so as not to allow the rope to be loosened and disengaged from the guide grooves in case of winding after the seat belt is suddenly withdrawn.
Accordingly, an object of the invention is to solve the aforementioned problems in the conventional retractor and to provide a seat belt retractor which is good in assembly and includes a compact tension reducing mechanism to achieve a high reliability in withdrawing the seat belt.
Further objects and advantages of the invention will be apparent from the following description of the invention.